The present invention relates to an ultraviolet-curable adhesive for bonding optical disks, and more particularly to an ultraviolet-curable adhesive composition which is used in producing information recording media by applying it to an information recording layer of an optical disk having the information recording layer on one surface and bonding thereon another optical disk which may have an information recording layer.
Production of optical disks (hereinafter referred also to as xe2x80x9cDVDxe2x80x9d) used as an information recording medium has been made in such a manner as applying an adhesive to an information recording layer of an optical disk having the information recording layer on one surface and laminating onto the information recording layer another optical disk which may have an information recording layer and curing the adhesive.
Ultraviolet-curable adhesives are suitable for use in this method from the viewpoints of simplification of DVD production steps, no occurrence of warping of disks owing to heating of the disks. In that case, it is required for curing the adhesive that ultraviolet rays irradiated from the outside to disks having an information recording layer and a protective layer and laminated with the adhesive pass through the substrate, the information recording layer, the protective layer and the like. For this reason, it was recognized difficult to make ultraviolet rays reach the adhesive to cure it. Thus, it was popular to use hot melt adhesives or thermosetting adhesives.
However, hot melt adhesives have the problem that the adhesives are easy to melt again if heat is applied to DVD using them, so the mechanical strength cannot be retained at high temperatures. Also, in case of producing DVD by using thermosetting adhesives, problems such as warping, deformation and the like owing to heating arise.
Thereafter, as a result of investigation in every direction about the use of ultraviolet-curable adhesive, there are proposed ultraviolet-curable adhesives which can be cured even by a slight amount of ultraviolet rays reached passing through the substrate, information recording layer, protective layer and the like. For example, known are an ultraviolet-curable adhesive of radical polymerization type which contains a (meth)acrylic monomer or oligomer as a main component and a photopolymerization initiator such as a benzoyl alkyl ether compound, a benzophenone compound or a thioxanthone compound, an ultraviolet-curable adhesive of polyene-polythiol addition polymerization curing type, and an ultraviolet-curable adhesive of cationic polymerization type comprising an epoxy resin and a Lewis acid type photopolymerization initiator (Japanese Patent Publication Kokai No. 8-161771).
However, since a thin film of a metal such as aluminum is used in the information recording layer and is easy to be attacked by a cationic polymerization type adhesive, ultraviolet-curable (meth)acrylic adhesives of radical polymerization type are popularly used at present.
Most of photopolymerization initiators to be used in the ultraviolet-curable (meth)acrylic adhesives have an absorption also in a high wavelength region. In general, initiators having an absorption at a wavelength of 360 nm or more and whose molar extinction coefficient in that wavelength region is as large as possible are used (Japanese Patent Publication Kokai No. 9-31416, No. 9-35335, No. 9-169956 and No. 10-8018).
However, if an optical disk wherein the adhesion is performed by the ultraviolet-curable (meth)acrylic adhesive of radical polymerization type is allowed to stand in a high humidity atmosphere, there arises a problem that moisture is easy to be adsorbed by the adhesive layer and the adsorbed moisture stays at the interface between the adhesive layer and the disk, and also a component in the adhesive migrates, so these adversely affect the adjacent information recording layer to remarkably lower the recording performance. A translucent film (vapor deposition film) of Al, Au, SiN, SiC, Si or the like is generally used as the information recording layer. The thickness of the film is usually from 50 to 300 angstroms. In case of aluminum deposition film, the thickness is from 400 to 1,000 angstroms. If the translucent films become thick, the bad influence of moisture on the recording performance of these translucent films particularly increase. Also, since a component itself of the photopolymerization initiator may corrode the metal thin film depending on the kind of the initiator, the bad influence on the information recording layer further increases in that case.
An object of the present invention is to provide an ultraviolet-curable adhesive which is hard to cause staying of moisture in the interface between the adhesive layer and the information recording layer even in the case that a relatively thick translucent film is used, thus scarcely exerting a bad influence on the information recording layer, and which does not cause separation of the laminated disks.
In general, an adhesive using a nitrogen atom-containing monofunctional monomer such as N-vinylpyrrolidone has a high water absorption and is easy to introduce a problem in various uses. For such a reason, in general N-vinylpyrrolidone has not been used for applications requiring a moisture resistance.
The present inventor has found that an ultraviolet-curable composition containing N-vinylpyrrolidone which is a nitrogen atom-containing monofunctional monomer, and a specific urethane (meth)acrylate oligomer as essential components has, contrary to expectations, a good moisture resistance and is useful as an adhesive for bonding optical disks which requires a moisture resistance, and exhibits excellent characteristics as compared with conventional adhesives.
Thus, in accordance with the present invention, there is provided an ultraviolet-curable adhesive for bonding optical disks, which is characterized in containing:
(A) N-vinylpyrrolidone,
(B) a urethane (meth)acrylate oligomer derived from a caprolactone-based polyol, an alicyclic diisocyanate and (meth)acrylic acid, and
(C) a photopolymerization initiator containing (C-1) a photopolymerization initiator composed of only a carbon atom, a hydrogen atom and an oxygen atom without containing a nitrogen atom, a sulfur atom and a phosphorus atom and (C-2) a photopolymerization initiator containing a nitrogen atom and/or a sulfur atom in a (C-1)/(C-2) ratio of 70/30 to 100/0 by weight.
The photopolymerization initiator (C) may be composed of only the component (C-1) or may be a combination of the components (C-1) and (C-2).
The adhesive of the present invention may further contain (D) 5 to 100 parts by weight of an alicyclic mono(meth)acrylate and/or (E) 5 to 95 parts by weight of a bisphenol type di(meth)acrylate per 100 parts by weight of the total of the components (A) and (B), provided that the amount of the component (E) does not exceed the amount of the component (B).
The adhesive of the present invention satisfies such characteristics required for adhesives for bonding optical disks that it has an adequate softness, structural destruction of a cured resin does not occur even if subjected to a moisture resistance test, the adhesion property to disk materials is not lowered, and the performances of a metal film are not lowered. Also, since the adhesive contains N-vinylpyrrolidone and a specific urethane (meth)acrylate oligomer as the constituent components, the adhesive layer has a high water retainability and accordingly is hard to cause problems such as staying of the absorbed moisture in the interface between the information recording layer and the adhesive layer and deterioration of the adhesion property, thus exhibiting an excellent moisture resistance.